Messages in this thread | | | Date | Tue, 1 Sep 2015 11:59:23 +0200 | From | Oleg Nesterov <> | Subject | Re: wake_up_process implied memory barrier clarification |
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On 09/01, Boqun Feng wrote: > > But I'm still a little confused at Oleg's words: > > "What is really important is that we have a barrier before we _read_ the > task state." > > I read is as "What is really important is that we have a barrier before > we _read_ the task state and _after_ we write the CONDITION", if I don't > misunderstand Oleg, this means a STORE-barrier-LOAD sequence,
Yes, exactly.
Let's look at this trivial code again,
CONDITION = 1; wake_up_process();
note that try_to_wake_up() does
if (!(p->state & state)) goto out;
If this LOAD could be reordered with STORE(CONDITION) above we can obviously race with
set_current_state(...); if (!CONDITION) schedule();
See the comment at the start of try_to_wake_up(). And again, again, please note that initially the only documented behaviour of smp_mb__before_spinlock() was the STORE - LOAD serialization. This is what try_to_wake_up() needs, it doesn't actually need the write barrier after STORE(CONDITION).
And just in case, wake_up() differs in a sense that it doesn't even need that STORE-LOAD barrier in try_to_wake_up(), we can rely on wait_queue_head_t->lock. Assuming that wake_up() pairs with the "normal" wait_event()-like code.
> which IIUC > can't pair with anything.
It pairs with the barrier implied by set_current_state().
Oleg.
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